This invention relates to a temperature sensing circuit for sensing the temperature of an active semiconductor device of a semiconductor body, for example, for sensing the temperature of a power MOSFET.
Various different forms of circuits for sensing the temperature of active semiconductor devices such as power MOSFETs have been proposed. Thus, for example, EP-A-341482 proposes providing a bipolar transistor in thermal contact with the active semiconductor device and using the increase with temperature of the current through the bipolar transistor to control the switching of a MOSFET which provides a signal indicative of an undesired temperature rise. EP-A-224274 adopts a different approach by providing a temperature sensitive polycrystalline silicon diode on top of the active semiconductor device.
The use of such temperature sensing circuits is of particular interest in the field of so-called `smart-power` devices in which control functions provided by low voltage logic devices are integrated with power semiconductor devices, such as power MOSFETs, to provide intelligent power switches for use in high-volume applications such as automative control systems.
It is desirable in such devices for the power device to be switched off or shut down if it overhears because, for example, of a short-circuit. However, in order to provide high current handling capability power semiconductor devices are generally rather large area devices and in certain overload conditions, for example, if a low resistance, say about 100 m.OMEGA. (milli-ohm), load is connected between ground and the power semiconductor device, the temperature at the centre of the power semiconductor device can rise rapidly whereas the temperature of the semiconductor body surrounding the power semiconductor device may rise considerably more slowly. In such circumstances, the provision of a temperature sensitive device in thermal contact with the power semiconductor device may not provide a sufficiently accurate or rapid response to the change in temperature of the power semiconductor device. Also, where the temperature sensitive element is provided on top of the active semiconductor device, the accuracy of the temperature sensing may be adversely affected, especially if the thermal contact to the active semiconductor device is not particularly good.